FIG. 5 shows a conventional suction system for supplying suction air to an internal combustion engine for a vehicle. In the drawing, 2 represents an internal combustion engine, 4 an air cleaner, 6 a suction air duct, 8 a suction air case, 10 a carburetor, and 12 an intake manifold. A filter element 16 is installed in the air cleaner case 14 of the air cleaner 4. The air taken into the air cleaner case 14 through the filter element 16 passes through the suction air duct 6, a component of the suction air passage, and the suction air passage 18 of the suction air case 8, and is mixed with fuel in the carburetor 10 to produce an air-fuel mixture which is supplied to each combustion chamber (not illustrated) of the internal combustion engine 2 by the intake manifold 12. The above suction system is disclosed in Japanese Publications Nos. 62-14162 and 62-171657. The suction system disclosed in the former publication is designed so that the inside diameter of the suction air passage will be changed according to the suction air temperature, while that disclosed in the latter publication is equipped with a rectifying unit to rectify turbulence of suction air at the joint between the air cleaner and the suction air duct and with a mesh cover o covering the upstream side of the above rectifying unit.
In the conventional vehicle suction system, however, there is a problem of making the driver feel uncomfortable because the suction air duct is vibrated due to the pulsation or turbulence of suction air to produce air suction noises which leak from the air cleaner to the outside and increase the noises inside and outside the vehicle. Therefore, a resonator is conventionally installed in the suction air passage to decrease noises inside and outside the vehicle by muffling air suction noises. However, the resonator does not adequately decrease the noises because it cannot rectify the pulsation and turbulence of suction air or inhibit air suction noises from being propagated to the air cleaner. The resonator also has the disadvantage that it can easily be incorrectly installed because the proper installation direction is difficult to determine by the appearance of the resonator and the weight of the resonator makes it difficult to handle.